For floating gate (Floating Gate)-type non-volatile semiconductor storage device, a desired device can be realized by using a 3-dimensional cell structure. More specifically, a floating gate-type non-volatile semiconductor storage device with a 3-dimensional cell structure has a structure formed by sequentially forming a tunnel insulating film, a floating gate (FG) material used as a charge storage film, an IPD (Inter Poly Dielectric) film, and a CG (Control Gate) one above the other on a semiconductor substrate. In the resulting device, the IPD film covers not only the top surface of the floating gate but also the sidewalls thereof. However, when the distance between adjacent memory cells in the non-volatile semiconductor storage device is reduced, it becomes difficult to provide a space between the control gate of the memory cells in the 3-dimensional cell structure described above. Therefore, a flat cell structure with an IPD film having a flat structure formed on the top surface of the floating gate has been proposed. In the related 3-dimensional cell structure, when the IPD film is formed into a structure that also covers the sidewall parts of the floating gate, the area of the IPD film is increased, and the capacitance ratio between the tunnel insulating film and the IPD film is increased. In the flat cell structure, however, since the area ratio of the tunnel insulating film to the IPD film is almost 1, it is difficult to increase the capacitance ratio. Therefore, it is difficult to apply sufficient electric field to the tunnel insulating film during writing/erasing.
Therefore, as one of the methods for improving the writing/erasing characteristic of the non-volatile semiconductor storage device having a flat cell structure, a structure having two floating gate layers (double floating gate type cell) is proposed. This double floating gate-type cell structure has three kinds of insulating films, that is, a tunnel insulating film formed on a semiconductor substrate, an IFD (Inter Floating-Gate Dielectric) film formed between the two floating gate layers of, and an IPD film formed between floating gate and control gate layers. More specifically, this IFD film is an insulating film used for insulating between the floating gate formed into two layers. Since the double floating gate-type cell has two layers of floating gate, charges can also be stored in the upper floating gate that is further away from the semiconductor substrate. Also, since the upper floating gate is in contact with the semiconductor substrate via two insulating films, the stored charges are difficult to extract compared with the charges stored in the lower floating gate that is closer to the semiconductor substrate.